1. Field of the Invention
The present invention relates in general to electric powered vehicles (EVs), and more particularly to a new, improved drive train technology for these vehicles.
2. Description of Related Art
There are currently many different types of highly efficient electric motors available for use in EVs. Nevertheless, in any given electric motor, at a given voltage, high efficiency may only exist in the upper speed range of the motor at light loads. As the load (required torque) is increased above this range, the required current increases in proportion to load, but even with a constant voltage source the speed of the motor decreases rapidly with the increase in load. Therefore, the required electrical input power to the motor (the current multiplied by the voltage) will increase at a greater rate than the mechanical output power from the motor (the torque multiplied by the speed of the motor). This results in a continuing loss of efficiency (output power divided by input power) as the load increases and the speed decreases. The efficiency of these types of electric motors used on EVs is highest when it is at higher speeds under lower loads and lowest when at low speeds under high loads, gradually dropping down to zero at stall.
If driven correctly, EVs with manual transmissions and enough gear changes can be operated more efficiently over a wide range of speeds, for example, by shifting at the right time to consistently keep the motor at high speed. But, perhaps based on habits learned from operating vehicles with manual transmissions and internal combustion (I.C.) engines, most drivers do not shift manual transmissions on EVs correctly; they shift too soon keeping the motor at low speed and high load. The efficiency characteristics of I.C. engines are generally opposite to those of the electric drive motors previously discussed; the efficiency of most I.C. engines is highest under high loads and medium speeds, and lowest under low load and higher speeds. There is a strong tendency for drivers to manually shift EVs like their I.C. counter parts, which reduces the efficiency and can overheat the electric motor and its controller.
Multi-speed transmissions like the ones used in today's I.C. engine vehicles with five or six constant mesh gears and corresponding syncromesh rings and clutches create a significant amount of drag and lower the range of an electric powered vehicle. The automatic versions with hydraulic torque converters are even less efficient and can be too inefficient to use in EVs.
It is desirable to have an electric vehicle drive system that automatically changes the ratio of the speed of the driving motor(s) to the drive wheel/s to keep the motors of the drive system operating more efficiently.